1. Field of the Invention
This invention relates to a driving unit for an electric motor driven bicycle and, in particular, to a speed reducing unit for a bicycle with an auxiliary drive and a pedaling torque detector contained in the reducing unit.
2. Description of the Prior Art
For example, a conventional auxiliary power unit for a bicycle is disclosed in JPA-56-149277 and will hereinafter be described with reference to FIGS. 1 and 2. As illustrated in the figures, the bicycle has a rear hub 1 and first and second free wheels 2 and 3 coupled thereto. The first free wheel 2 is connected to a driving chain sprocket 4 through a first chain 5. The second free wheel 3 is connected through a second chain 8 to a chain sprocket 7 coupled to an output shaft of a driving motor 6.
To the intermediate portion of the first chain 5, a torque detector 9 is attached which is for detecting the magnitude of tensile strength applied to the first chain 5 to convert the magnitude into an electric quantity (for example, electric resistance or electric capacitance). Upon running the bicycle, pedaling force is produced by a rider and transmitted through pedals 14, pedal cranks 15, and the driving chain sprocket 4 to the first chain 5 which is then subjected to the tensile strength. The tensile strength is detected as a rotation angle of a rotary shaft of the torque detector 9.
The rotary shaft of the torque detector 9 is rotated by the rotation angle corresponding to the pedaling force. Upon start, acceleration, or hill climbing, the pedal 14 is strongly kicked and the pedaling force is increased. When the rotary shaft of the torque detector 9 is rotated by an angle greater than a predetermined angle, the driving motor 6 is energized to rotate the chain sprocket 7 coupled to the output shaft of the driving motor 6. The rotation of the chain sprocket 7 causes the rotation of the second free wheel 3 through the second chain 8 to drive a rear wheel. Thus, the rear wheel is driven by a combination of pedaling torque by human power and rotation torque by the driving motor 6.
In the conventional auxiliary power unit described above, the second chain 8 is wound around an output end thereof, i.e., the output shaft of the driving motor 6. With this structure, the power unit inevitably protrudes beyond the width of the bicycle and the extent of protrusion is not small. In addition, the output shaft is cantilevered and, therefore, easily bent. It is therefore difficult to keep a balance between left and right sides of the bicycle. Since the power unit is located near to the rear wheel, weight distribution is too large on the rear wheel and too small on a front wheel. This often results in unstable running operation of the bicycle.
In view of the above, the auxiliary power unit is desired to be compact in its axial direction (widthwise direction of the bicycle), to keep good balance in weight distribution between the front and the rear wheels, and to be easy in handling. To satisfy such requirements, various proposals have been made. For example, JP-A 40878/1995 discloses an electric driving power unit arranged coaxially with a crank shaft. An electric motor is located frontward of the crank shaft to improve the balance in weight distribution of a body of the bicycle. The power unit contain pedaling torque detecting means and a one-way clutch. However, this electric driving power unit is still disadvantageous in view of weight and cost.
Generally, an electric motor driven bicycle having an auxiliary power unit is mainly driven by human power and is desired to provide a rider with the feeling of an ordinary pedal-driven bicycle. Therefore, the power unit is required to be light in weight, low in cost, and easy in operation of a control system.